HORMONES 2010, 9(1): -15
DOI: 
Mini Review
Definition, classification and mechanism of action of endocrine disrupting chemicals
Wolfgang Wuttke, Hubertus Jarry, Dana Seidlovα-Wuttke

Department of Endocrinology, University Medical Center Goettingen, Georg-August-University, Germany

Keywords

Bisphenol-A, Endocrine disruptors, Fungicides, Herbicides, Isoflavones, Pesticides, Phtalates


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Hormones are peptides/proteins, lipids or aminoacid derivatives which after their secretion act on receptors to cause reactions within the respective receptive cells. Each cell of the body is in one way or another regulated by hormones. Hence, hormones exert innumerable effects on the organism of animals and humans.

These multiple effects can be adversely deregulated by Endocrine Disrupting chemicals, (EDs), which are taken up through food or drinks or through the air. Occasionally, they are also absorbed transdermally. This topic has been dealt with in an Endocrine Society scientific statement.1 EDs are mainly unintentionally taken but there are also examples of intentional intake of substances possessing putative ED properties.

Basically, hormones vitally modify three biological processes: they are essential for reproduction, for optimal survival and for normal biological performance. Hence, they are essential for the maintenance of species and for coping with daily environmental influences.

Most EDs may interact with the effects of lipid (steroid) or aminoacid derived (thyroid) hormones, while a few interact with peptide/protein hormone synthesis or signalling. However, effects of EDs through steroid receptors on peptide/protein hormones are common. Some examples of action of EDs are given in Tables 1 and 2 .

Other less well explored mechanisms of action of EDs are direct effects on genes. Estrogens and EDs with estrogenic action were, for example, suggested as causing DNA damage, thereby promoting malignant differentiation of affected cells.2 A troubling new aspect is their epigenetic impact; the amount of methylation of genes occurring early in life may have profound effects years later3,4 and may even be transgenerationally inherited.5

Most EDs interfere with reproduction. They act as either agonists or antagonists of the steroidal sex hormones, estrogens or androgens. A number of EDs interfere with the daily necessity of coping with internal or environmental stress and other adverse events. Fortunately, very few EDs exert a life-threatening impact through interference with hormone systems necessary to maintain basic life-sustaining mechanisms. Nevertheless, the effects of some EDs may be life-threatening because they may interfere with the normal functions of organs or cause malignancies of these organs.

There follows a discussion concerning the basic mechanism of EDs actions, some of which are detailed in the Tables.

REPRODUCTION

The effects of EDs on reproduction are amply documented in both marine and terrestrial species, and in humans. Maintenance of species is dependent on an integrated interplay of the entire endocrine system. Reproduction ceases when life-supporting hormonal systems are disturbed, but “performance”-supporting hormones are also needed to maintain normal reproduction.

Of major importance for reproduction is normal functioning of the reproductive system, i.e. the hypothalamo-pituitary-gonadal axis, as well as the proper functioning of other organs regulated by gonadal steroids, such as the uterus and to some degree the mammary gland. Most EDs that disrupt reproduction interfere with steroidal signalling, i.e. with the proper action of estrogens or androgens. Some EDs either inhibit synthesis or interfere with the metabolism of sex steroids. Most EDS with adverse effects on reproduction bind either to the estrogen receptors (ERs) or to the androgen receptor (AR) and, by doing so, may either stimulate or inhibit the transcriptional or post-transcriptional mechanisms; moreover, steroid receptors located in the membrane, affecting either ion channels or 2nd messengers, may mediate ED effects. By means of these mechanisms, they may interfere with fundamental sex steroid effects on the brain, the pituitary gland, the gonads and the accessory sex organs, such as the uterus and mammary gland in females and the prostate and seminal vesicles in males.

The classical endocrine disruptors are the dioxins, which bind to the arylhydrocarbon receptor (AhR; formerly also called dioxin receptor). After dimerizing with the arylhydrocarbon receptor translocator (ARNT), the resulting transcriptional activity increases the activity of the cytocrome P 450 1A1 gene resulting in higher production of CYP 1A1, an enzyme involved in detoxification of a number of environmental substances.6,7

Of major concern is the fact that endocrine disruptors interfering with reproduction may have profound effects on sexual differentiation. There have been many examples demonstrating disturbed sexual development of animals in areas polluted with EDs.8-10 The most dominant endocrine disruptors interfering with reproduction and sexual differentiation have either estrogenic or antiestrogenic effects. There is good evidence that the consumption of oral contraceptives containing ethinylestradiol contaminates waste water. This strong estrogen is unmetabolized excreted through the urine and its clearance through bacteria in waste water is incomplete. Therefore, ethinylestradiol is present, in biologically active concentrations in waste water drainages into rivers and this has profound effects on aquatic life in these areas.11-13

Pollution of surface water with pesticides, fungicides or herbicides is also of great concern since most of them have relatively strong antiandrogenic effects.1 Through incorporation into aquatic life forms, these substances may also enter the human and animal food chain. In addition, the antiandrogenic substances used in pesti-, fungi- or herbicides can directly contaminate our daily food.

Both estrogens and antiandrogenic compounds in surface waters have profound effects on aquatic life. Feminisation or demasculinisation of molluscs, arthropods and fish have been reported in polluted lakes or rivers (Lake Apopka).14,15

Lately, the endocrine disrupting effects of bisphenol-A has raised great concern. In the United States discussions are taking place as to whether bisphenol-A should be used as a plasticizer. In animal experiments, minute amounts were effective in causing adverse effects, whereas these effects were not exerted by larger amounts of this plasticizer. It remains unclear why minute amounts of this compound are more effective in eliciting endocrine disrupting effects than higher doses.4,16-19 It has been claimed that bisphenol-A acts on estrogen receptors.

Concern has also recently been expressed regarding the putative endocrine disrupting effects of some substances protecting the skin against ultraviolet radiation.20,21 Some of these UV screens have proved to exert estrogenic effects: benzophenone 2, in particular, was almost as active as the native estrogen estradiol-17ß, primarily by addressing estrogen receptor alpha signalling.22,23  Benzophenone-2 has therefore been banned as a UV screen for human use, especially since it was also shown to penetrate the skin and circulate in serum. Additionally, other UV screens such as 4-methylene-benzolidin-camphor (4-MBC) and some cinnamates have mild estrogenic effects and their mechanism of action involves activation of estrogen receptors and recruitment of cofactors.24

As already mentioned, most endocrine disruptors are present as ingestible or inhalable chemicals which are often unwittingly taken up. There are, however, endocrine disruptors which are intentionally taken up like phytoestrogens which the Environmental Protection Agency (EPA) considers to be potential endocrine disruptors.

According to a list of endocrine disruptors published by the EPA, isoflavones, present in a variety of soy-derived foodstuffs, have relatively strong estrogenic effects. Their presence in baby food is of particular concern. Moreover, a large number of food additives is present on the market for the treatment of climacteric and postmenopausal complaints or diseases. This results in many women, suffering from climacteric dysfunctions, ingesting high amounts of highly purified isoflavones. It is currently being debated whether the estrogenic properties of these high amounts of isoflavones may have adverse effects on the mammary gland or the uterus.

These isoflavones are intentionally or unintentionally administered, as, for example, when parents feed their babies soy-containing baby food. Soy is the major protein source in many baby food formulas. It contains estrogenic isoflavones which are absorbed through the infant’s gastro-intestinal tract and, in fact, measurement of isoflavones and their metabolites in the urine of babies has revealed that large quantities are present in infants’ urine. In table 3 , the average intake of isoflavones by Japanese women and US American vegetarians are compared with the isoflavone uptake through a soy-containing baby food formula. Serum concentrations of isoflavones in Japanese women were in the range of 10-6M to 10-8M,25 those in soy formula fed babies amounted to 10-5M, and in the urine were even higher.26 Hence, amounts of isoflavones in some baby formulas exceed those taken up by the average Japanese (on the basis of per kg BW) by a factor of >10 (Table 3). High amounts of isoflavones given to newborn rats resulted in disturbance of estrous cyclicity at a very early stage of life.27

Experimental data from cell cultures and whole animal studies showed that such concentrations had strong estrogenic effects.28 Furthermore, they may imprint many organs, including the brain, to develop diseases much later in life. Severe disturbances of high doses of isoflavones on reproduction were demonstrated in mice and rats.29,30 Another distressing observation is that some isoflavones are inhibitors of topoisomerase II. When such inhibition occurs during fetal life, an increased risk for later development of leukemia is anticipated.31 Various adverse effects of prenatal genistein (an isoflavone) treatment on the reproductive tract32 have also been reported. Such effects have not been proven and seem not to occur in the Japanese population. Isoflavones are also propagated as an alternative for hormone replacement therapy in women suffering from climacteric symptoms or in women at risk for the development of osteoporosis or arteriosclerosis. Companies producing soy/red clover/isoflavone-containing preparations for ageing women claim that their products have solely beneficial estrogenic effects without the putative adverse effects of estrogens in the mammary gland. In fact, it is often suggested that isoflavones may prevent mammary cancer. Consequently, the claim is made that the isoflavones are ideal selective estrogen receptor modulators. In recent reviews,33,34 however, the efficacy of these products to alleviate climacteric symptoms has been questioned. Since many patients believe “little helps little, more helps more”, they often take more than the recommended 50 mg quantity per day. In a large placebo-controlled study, the uptake of 150 mg of isoflavones per day had clear uterotrophic effects, since 3.37% of the postmenopausal women developed endometrial hyperplasia, whereas this was not observed in the placebo treated controls.35

The rationale of such claims for isoflavones is based on what is known as the “Japanese phenomenon”: Japanese women when living in Japan have significantly fewer mammary cancers than Caucasian American or European women, whereas when they migrate to the United States, the first daughter generation develop mammary cancer as often as do Caucasian women. This indicates that it is not genetic but rather environmental factors that are responsible for the differences in the development of mammary cancer. It soon turned out that the isoflavones present in soy food, which is the dominant protein source in Japanese nutrition, might exert the anticancer effects. After migrating to the United States, much more meat is consumed and, therefore, the isoflavone intake is minimized. On that basis, companies producing isoflavone-containing soy or red clover preparations claim that their product may prevent mammary cancers when given to climacteric/postmenopausal women. This concept has recently been challenged by the observation that carcinogen-induced mammary tumours in rats can be significantly prevented only by pre- and peripubertal administration of isoflavones, and particularly of genistein.36 This concept was confirmed by the observation that Japanese girls migrating to the United States prior to puberty have significantly higher risks for developing mammary cancers than Japanese migrating after puberty.37,38 Thus, it appears that the mammary cancer preventing effects of isoflavones are exerted at a crucial developmental period, namely the time of puberty, i.e. when mammary glands develop. This prompts the urgent question as to whether isoflavones, when given to “isoflavone inexperienced” women at the time of menopause, may have harmful effects.

Taken collectively, the effects of dietary isoflavones in newborns and climacteric/postmenopausal women are of great concern and need to be further investigated.

In a number of experiments, and particularly in experiments using estrogen receptor expressing cancer cells, the effects of minute amounts of substances known to have estrogenic effects potentiate each other, being not only additive but supra-additive. Therefore, a multitude of endocrine disrupting effects may be anticipated when combinations of endocrine disruptors are polluting our environment and are incorporated by plants or animals that are part of our food chain. Clearly, more work is necessary for a better understanding of the effects of endocrine disruptors in humans.

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Address for correspondence:
Prof. Dr. Wolfgang Wuttke, Department of Endocrinology,
University Medical Center Goettingen, Georg-August-University,
Robert-Koch-Strasse 40, D-37099 Goettingen/Germany,
Tel.: +49-551-396714, Fax: +49-551-396518,
E-mail: ufkendo@med.uni-goettingen.de

Received 10-11-09, Revised 12-12-09, Accepted 28-12-09


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